1. Field of the Invention
The present invention pertains to an apparatus for growing crystals of semiconductor materials for use in making integrated circuit devices and solar cells, for example. More particularly, the invention relates to such apparatus which operates in accordance with a Cochralski technique making use of a quartz crucible.
2. Prior Art
In a batch type apparatus for the growing of silicon single-crystal ingots, the quantity of a silicon melt contained in a quartz crucible decreases as the crystal grows. As a result, the quality is not uniform longitudinally within the crystalline ingot obtained by the apparatus of this type. More specifically, the oxygen concentration in the crystal depends upon the quantity of oxygen dissolved in the silicon melt, thereby varying as the silicon melt in the quartz crucible varies. In addition,since the quantity of the melt varies, the thermal distribution in the furnace fluctuates and the convection in the melt varies, so that the crystallization front varies. Further, dopant such as phosphorus, boron and antimony is generally added to the melt to control the electric conductivity of the single-crystal. However, insomuch as the segregation constant of such impurities is not identical to 1, the dopant concentration in the crystal differs as the crystal grows.
Thus, the quality of the single-crystal ingot varies in its longitudinal direction, and therefore it has been only a part of the ingot that has a desired quality. Additionally, the productivity has been low in such batch process.
The approach to resolve the above disadvantages is the use of a continuous type apparatus hitherto proposed wherein the single-crystal ingot is grown while the quartz crucible is being charged with a material. U.S. Pat. No. 2,892,739 or Japanese Patent application A-Publication No. 61-36197 describes one such apparatus which is the simplest in structure and makes use of the combination of a double crucible and the continuous charging of the material in the form of powders, lumps or granules.
In the above continuous type apparatus, however, the temperature of the silicon melt is above 1,420.degree. C., and the quartz of which the crucible is made begins to get softened at about 1,100.degree. C. As a result, although the outer crucible member supported by a graphite susceptor is not deformed from its original shape so greatly, the inner crucible member is susceptible to a large deformation since it is not supported sufficiently, so that the growing of the single-crystal is adversely affected. FIGS. 1 and 2 of the accompanying drawings schematically illustrate a conventional apparatus which comprises a quartz double crucible 100 housed in a susceptor 102 and consisting of inner and outer crucible members 100a and 100b, and a resistance heater 104 disposed so as to surround the crucible 100. The double crucible 100 is filled with a charge of silicon material 106, as shown in FIG. 1, and the charge is heated by the heater 104 until it is melted thoroughly. Each inner crucible member 100a, however, is subjected to deformation as shown in FIG. 2 as the silicon material is melted.